The aerodynamic drag on bluff road transport vehicle bodies consists mainly of pressure drag with skin friction and base drags forming insignificant parts of the total drag. The flow field of bluff bodies is usually characterized by a large wake and periodic vortex shedding. This is especially true of non-circular cylinders with sharp windward corners operating at low and moderate or subcritical Reynolds numbers. The Reynolds number associated with small and medium size road vehicles usually falls in the subcritical range (1.0 to 2.0.times.10.sup.6). The drag force associated with such flow pattern is very high. The existence of such a flow pattern over the road transport vehicles can lead to substantial expenditure of energy to overcome its aerodynamic drag.
One of the most popular systems or methods presently and previously employed for reducing the aerodynamic drag of bluff vehicles is the rounding of sharp corners or streamlining of the body shapes of the vehicles.
The effectiveness of corner rounding techniques, may be evaluated by utilizing a simple two-dimensional square section The drag coefficient of such a square section at subcritical Reynolds numbers is close to 2.0. With progressive increase in corner radius and creating a streamline effect, the drag coefficient reduces and reaches the limiting value close to 1.0 corresponding to circular cylinders. Thus, the maximum drag reductions achievable by the corner rounding technique appears to be about fifty percent. This limitation, along with the costs and other impractical aspects of the required substantial geometrical modification to the basic road vehicle to achieve this result, emphasizes the need for new systems and/or processes for aerodynamic drag reduction.